Typically, combustion apparatuses such as boilers are provided with an exhaust-heat recovery device to improve thermal efficiency. A known example of such an exhaust-heat recovery device is an economizer (supply water heater) for heating water to be supplied to a boiler in advance. An economizer is disposed in a duct through which exhaust gas flows, and exchanges heat with water to be heated such as boiler supply water, thereby recovering afterheat of exhaust gas. In particular, a condensation economizer is a latent-heat recovery device capable of recovering latent heat emitted when water vapor in exhaust gas becomes condensed and turns into liquid water. When a condensation economizer is provided along with a dry-type economizer for recovering mainly sensible heat of exhaust gas, thermal efficiency of a boiler improves even further. For instance, the latent-heat recovery device of Patent Document 1 is disposed in a duct through which exhaust gas flows as a downflow, at downstream of an exhaust heat (sensible heat) recovery device. Furthermore, the latent-heat recovery device is connected to a condensate circulation system of a steam turbine, and recovers latent heat of exhaust gas with condensate afterheat to improve power generation efficiency of a gas turbine combined cycle (GTCC).
Such a condensation economizer generally includes a heat transfer tube formed in a plurality of stages. Supply water flowing through the inside of the heat transfer tube exchanges heat with exhaust gas flowing through the outside of the heat transfer tube, and thereby supply water is heated by utilizing condensate latent heat of exhaust gas. That is, the water vapor contained in exhaust gas reaches the condensate temperature when passing through the heat transfer tube of the condensate economizer, and the state changes from water vapor to liquid water (condensate water). Thus, the heat transfer tube of the condensate economizer has a region (condensate region) formed therein, where condensate water is produced, while exhaust gas before reaching the condensate temperature passes through a section of the heat transfer tube upstream of the condensate region and forms a dry region. The position of the condensate region formed in the heat transfer tube fluctuates (moves) in accordance with fluctuation of the exhaust gas temperature caused by fluctuation of the boiler load, for instance, and thus a dry-wet alternation region is formed in the heat transfer tube, where a dry state and a wet state are repeated. That is, the dry-wet alternation region is a region in which cooling and heating in the heat transfer tube are repeated, thus being a region where stress corrosion cracking (SCC) of heat transfer tube may occur. To prevent SCC due to the dry-wet alternation region, in Patent Document 2, a condensate economizer is disposed in a duct through which exhaust gas flows as an upward flow, and is configured such that exhaust gas reaches the condensate temperature in the vicinity of the upper section of the condensate economizer. Accordingly, the entire heat transfer tube of the condensate economizer is humidified, and thereby generation of a dry-wet alternation region is prevented.